1. Field of the Invention
The present invention relates to a lithium secondary battery and a positive electrode of the lithium secondary battery.
2. Related Art
In recent years, a lithium secondary battery referred to as a lithium ion battery has widely been used for power sources of cellular phones and personal computers. Generally, a lithium secondary battery which has recently practically been used has a weight energy density of about 150 Wh/kg and is required to further densify weight energy density.
The lithium secondary battery in practical use employs a carbon-based material such as graphite for the negative electrode, a lithium-containing oxide such as LiCoO2 for the positive electrode, and an organic solvent, for example, cyclic carbonate such as ethylene carbonate, a chain carbonate such as dimethyl carbonate and the like, in which an electrolyte salt such as LiPF6 is dissolved, for the electrolyte solution. In such kind of lithium secondary battery, since lithium ion moves between the positive electrode and the negative electrode during charge and discharge, the energy density is determined depending on the specific capacity of positive electrode, the specific capacity of negative electrode, and the battery voltage.
The actual specific capacity of the carbon-based material to be employed for a negative electrode is 370 mAh/g in the case of the graphite which has the highest specific capacity and the actual specific capacity of LiCoO2, which is generally employed for a positive electrode, is about 150 mAh/g. As described above, comparing actual specific capacity between the positive electrode and the negative electrode, the capacity of the negative electrode is at least two times as high as that of the positive electrode and it is found effective to increase the actual specific capacity of the positive electrode rather than that of the negative electrode in order to increase the weight energy density of the battery.
However, a lithium-containing oxide such as LiCoO2 is known as a material whose crystal structure is broken and whose charge-discharge cycle characteristics are considerably deteriorated if Li is completely pulled out. Consequently, it is difficult to increase the actual specific capacity of the positive electrode using such as Li-containing oxide. Further, since cobalt reserves are in small amounts and thus cobalt is expensive, a positive electrode material in place of that is required.